The present invention relates to front end of line (FEOL) semiconductor processing, and more specifically to a FEOL method for oxide filling of trenches and optionally planarizing a substrate thereafter for subsequent processing.
As device sizes shrink and vertical transistors come into use, isolation trenches are being fabricated with increased aspect ratios (height to width) greater than 1:1, often exceeding 5:1. Such high aspect ratio isolation trenches are needed to fully block currents from moving within the substrate between active devices, especially devices having a vertically oriented channel which lie below the substrate surface and are close to each other. It is becoming more difficult to fill such high aspect ratio trenches and assure that voids are not left in the fill, despite the use of high quality filling processes such as high density plasma (HDP) oxide. Voids in the fill of an isolation trench can make a short circuit between conductor wires by trapping conductor material in the void during subsequent process steps.
Planarization poses another difficulty. The surface of a substrate must be planarized after isolation trenches are filled. However, some trench filling processes result in high and narrow width mounding of the deposited oxide above the surface of the substrate. Because of such topography, the horizontal forces in planarization by only chemical mechanical polishing (CMP) risks irreparably damaging the substrate by fracturing the substrate or pad material (e.g. pad nitride) which lies below the oxide mounds.
Accordingly, the present invention provides a method of forming and filling isolation trenches in a substrate. The method includes anisotropically etching trenches in a surface of a substrate and partially filling the trenches with a deposited oxide. As a byproduct of the partial filling, the oxide accumulates in mounds on the surface between trenches. The trenches are then filled with a supporting material, e.g. a highly flowable material such as polymer of the type commonly used in semiconductor processing as an anti-reflective coating (ARC), low-K dielectric, spin-on-polymer, or alternatively, a supporting material of polysilicon. When the supporting material is polysilicon, the oxide mounds on the upper sidewalls of the trench are first wet etched, and a liner of silicon nitride is preferably deposited prior to depositing the polysilicon. Once the supporting material is in place, a flattening process is applied to lower the topography of the mounds. The flattening process may include polishing with a fixed abrasive pad or, alternatively, chemical mechanical polishing and a directional (e.g. reactive ion type) etch. Thereafter, the supporting material is removed, and oxide deposition to fill the trenches is continued.
Preferably, the oxide is deposited by high density plasma (HDP). Preferably, the oxide filling of the trenches is performed by a series of alternating deposition and etchback steps. After the supporting material is removed and the trenches are completely filled with oxide, the substrate is preferably planarized by chemical mechanical polishing (CMP) with a slurry composition preferably including a ceria base.